1. Field of the Invention
The present invention relates to a pump assembly.
2. Background Information
Pumps are typically used to pump fluid through a hydraulic system. Pumps have a performance curve that characterizes the pump flow output at a predetermined back-pressure. There are different types of pumps which each have certain characteristics and advantages. For example, recreational vehicles typically have a diaphragm pump that pumps water from a storage tank to faucets, showers, etc. Diaphragm pumps are advantageous because such devices are self-priming, can run dry, and more efficiently generate demanded flow and pressure from the water system in a recreational vehicle. The pump and motor are typically sized to meet the maximum anticipated demand of the water system. By way of example, the maximum demand in a recreational vehicle may occur when all of the faucets are open.
The diaphragm pump is driven by a motor coupled to a pressure switch that senses the pressure within the water line. The pressure switch is typically designed to turn on at a low pressure and turn off at a higher pressure.
When the water pressure falls below a threshold value the pressure switch activates the motor to drive the pump. The pump then pumps water according to a pump performance curve shown in FIG. 1. As shown in FIG. 1, the range of flowrates between the on and off pressures is relatively limited. When the demand for water is less than the minimum flowrate, the pump will cycle between on and off states to maintain the water pressure within the system. Cycling reduces the life of the pump. Cycling also creates undesirable fluctuations in flow. For example, the pump may be in a water system where a cold faucet and a hot faucet are partially open. Given different dynamics of each line, the flow fluctuations created by a cycling pump may create undesirable variations in water temperature.
Some systems incorporate accumulators that can store the output of the pump and reduce the number of pump cycles. Acculumators are bulky and add to the cost of the system.
Some diaphragm pumps include by-pass valves that allow continuous pump operation when the line pressure has reached a desired level. Such an approach is not energy efficient because as actual demand decreases, an increasing amount of energy is required to re-circulate water within the pump. It is also difficult to reliably generate the higher pressure needed to deactivate the pressure switch when there is no demand for water.
Most water pumps are positive displacement devices that theoretically generate the same flowrate regardless of the line pressure. To insure that water can be provided to all of the faucets, etc, the pump is configured to always operate at a maximum power given a maximum flowrate. The hydraulic system does not always need the maximum flowrate. There is an inefficiency in operating a pump in this manner. It would be desirable to provide a positive displacement pump that can operate continuously over a wide range of flows and vary the pump output as a function of the line pressure within the system.
Additionally, the prior art pumps start up at full power and turn off at full power. Starting and stopping at full power can create a shock in the system (waterhammer). This shock stresses the system and may produce an undesirable audible noise. It would be desirable to provide a pump that ramps up to a desired flow and gradually reduces power before turning off.
One embodiment of the present invention is a pump assembly that includes a pulse width modulator circuit. The pulse width modulator circuit generates a series of pulses that drive a motor. The motor drives a positive displacement pump that creates an output pressure. The circuit can sense variations in the motor current of the motor and change the energy provided by the pulses as a function of the varying current. A pressure switch activates and deactivates the pulse width modulator circuit.